1. Field of the Invention
This invention relates generally to the field of acoustic tumor detection. More particularly, the invention relates to a diagnostic technique for detecting small (.ltoreq.1 cm) intracanalicular tumors.
2. Prior Art
Until recently, the auditory brainstem response (ABR) test was an important component of the clinical test battery for acoustic neuromas. Early studies reported detection in the 95% -98% range, but these tumors were typically fairly large. In "Electrocochleography and Auditory Brainstem Electric Responses in Patients with Pontine Angle Tumors", Ann Otol Rhinol Laryngol 1980;89 (suppl 75):1-19, J. J. Eggermont, M. Don, and D. E. Brackmann examined the impact of tumor size on detection using estimates from CT scans and surgical reports and concluded that tumors smaller than 1.0 cm often go undetected by standard clinical ABR methodology. This conclusion has been supported by more recent and extensive studies that compared the sensitivity of this ABR methodology with gadolinium (Gd-DTPA) enhanced magnetic resonance imaging (MRI). Using standard measures of peak and inter-peak latencies, waveform morphology, or presence of waves, the ABR tests were nearly 100% accurate in detection of all extra- and intracanalicular tumors larger than 1.0 cm. However, for intracanalicular tumors smaller than 1.0 cm, the accuracy of some of these standard latency measures varied across studies from 63% to 93% with corresponding false negative rates ranging from 7% up to 37%. This wide range of detection and false positive rates is probably due to the different criteria selected. The high failure rate in detecting small intracanalicular tumors in some of these studies is not surprising since normal ABR latencies are possible if they are determined by a frequency region of the cochlea not affected by the tumor. Possible mechanisms underlying latency changes caused by tumors have been discussed and reviewed in "Efficacy of Auditory Brainstem Response as a Screening Test for Small Acoustic Neuromas", Am J Otol 1995; 16:136-139 by M. L. Gordon and N. L. Cohen.
ABR Methodology
Two of the most widely used ABR latency measures for tumor detection, the IT.sub.5 (Inter-aural Time for wave 5) and I-V interpeak delay, are illustrated in FIG. 1. The top trace represents a standard ABR from a normal-hearing non-tumor ear; the lower trace, the response from an ear with a tumor. The IT.sub.5 is the wave V latency difference between ears of the same subject. If the difference is greater than 0.2 ms after compensating for hearing loss, the test is positive for a tumor. While this criterion has been effective in detecting tumors larger than 1.0 cm, its sensitivity to smaller tumors is only about 80%. Furthermore, the IT.sub.5 should not be used when bilateral tumors are suspected, as in Neurofibromatosis II (NF2) cases, or if the non-suspect ear has a greater than moderate degree of hearing loss and poor ABRs. Thus, this measure is most useful in detection of unilateral tumors when there is relatively little impairment of the non-suspect ear.
The other latency measures illustrated in FIG. 1 are the interpeak delays between waves I and V and between waves I and III of the ABR from the ear suspected of a tumor. In males, the I-V delay is normally about 4.2 ms and the I-III delay is about 2.1 ms; in females the delays are slightly shorter. The advantage of these delay measures is that they are intra-aural; thus, it is not necessary to consider the hearing status of the subject's non-suspect ear. If the delays are abnormally long, the test is positive and a tumor is suspected. The accuracy of these measures depends on the definition of "abnormally long". Some studies used two or three standard deviations from the mean for normal-hearing subjects, while others used a specific delay, e.g. I-V delay of 4.4 ms or greater. Many of these studies have shown that small intracanalicular tumors are difficult to detect using latency measures. The major problem in measuring interpeak delays is the difficulty of recording wave I in patients with tumors or high frequency hearing losses. Other latency measures, such as the inter-aural rather than the intra-aural I-V delay and latency-intensity functions, have also been examined.
ABR latencies are typically used in clinical applications because they are robust measures. By definition, a robust measure tends to be insensitive to small perturbations. Thus, it is not too surprising that robust latency measures are insensitive to the small neural perturbations caused by small tumors. A more detailed explanation of this insensitivity is presented below.
Consequence of ABR Insensitivity to Small Tumors
Since many studies have demonstrated the inadequacy of standard ABR measures in detection of tumors smaller than 1.0 cm, clinical practice has shifted to using MRIs. Gd--DTPA MRIs have now replaced contrast computed tomography (CT) as the "gold standard" in diagnosis of acoustic tumors. MR imaging does not use ionizing radiation and provides greater soft tissue contrast which improves differentiation of tumor from surrounding bone, spinal fluid, and brain. In addition, Gd--DTPA MRI provides reliable identification, size, and location of tumors as small as 3.0 mm in the internal auditory canal region.
Unfortunately, due to the low incidence of acoustic tumors even in cases with clinically significant signs, many patients without tumors are imaged. Changes in the health care delivery system now place a heavy emphasis on cost reduction and justification of expensive medical tests.
One approach to reducing MRI costs involves reduced field studies in which a limited number of slices are taken through only the cerebellopontine angle (CPA), thereby excluding the rest of the brain. Typically, the requests for these limited studies are very specific and not of a general screening nature. While this approach risks overlooking other lesions of the intracranial vault because of the limited field of view, a number of other specialties that rely on imaging perform limited studies without undue concern about the medical-legal liability of missing nearby lesions. This concept of limited studies could be a viable alternative to the full MRI when specifically searching for a mass lesion of the CPA.
A second approach, using new surface coils technology capable of rapid data acquisition and faster studies, appears to have good sensitivity. The faster studies result in shorter scanning time and, therefore, less cost to the patient. However, this faster technology requires major software and hardware upgrades for older systems. If upgrades are possible, the cost may be as much as $1,000,000. Newer systems are easier to upgrade and have incorporated hardware that reduces the scanning noise of fast acquisition studies. The availability and distribution of these new faster systems are unknown.
Patient-related problems sometimes limit the usefulness of the MRIs. For example, many individuals cannot tolerate the claustrophobic confines of the scanner. Some of these individuals may require extraordinary measures such as sedation to even attempt an MRI. Others are simply too large to fit into the MRI machines. Open MRI systems may reduce these problems, but currently the resolution of these systems is sub-optimal for detecting small acoustic tumors. In addition, some individuals may have metal (such as pacemakers) in their bodies that cannot be exposed to the magnetic field. Finally, small communities may not have access to an MRI device. The cost and inconvenience to travel long distances to an imaging center may be problematic for some patients.
The emphasis on medical cost containment may require additional justification for requesting an expensive imaging test. A significant inroad to medical cost containment and justification could be achieved by an improved ABR methodology that detects small acoustic tumors with high sensitivity and acceptable specificity. Standard ABR techniques are relatively inexpensive, non-invasive, non-threatening, and widely available.
The Search for Another ABR Measure
If latency is often insensitive to the effects of small tumors, what about other parametric measures of ABRs? Over the years, many studies examined ABR amplitude measures and concluded that they are often too variable when compared to latencies. However, amplitude measures should be very sensitive to loss or desynchronization of eighth nerve activity due to compression by an acoustic tumor. In "The Auditory Brain Stem Response I-V Amplitude Ratio in Normal, Cochlear, and Retrocochlear Ears", Ear Hear 1984; 5:52-55, Musiek, Kibbe, Rackliffe, et al. used wave V to wave I amplitude ratios and found that low ratios (less than 1) indicated the presence of a tumor; normal-hearing and cochlear lesion patients seldom had ratios lower than 1. However, only 44% of the tumors were detected in this manner. Other problems with these amplitude ratios are: 1) they are dependent on the presence of wave I; 2) they may be more sensitive to variations in the audiometric configuration because the amplitude of wave I is more dependent than wave V on higher frequency regions of the cochlea; and 3) using a ratio of two measures that are highly variable results in an even more highly variable measure.